|M.Sc Student||Cabri Oshrat|
|Subject||Spectral Assignment and Slow Dynamic Processes by Solid|
State NMR: Methodology Development and Application
to Polymorphism and Phase Transition in
|Department||Department of Chemistry||Supervisor||Professor Asher Schmidt|
In this work, we have developed a new pulse sequence, dbFSR, for the purpose of frequency-selective reintroduction of the dipolar interaction. This pulse sequence enables us to obtain dipolar recoupling to nuclei with chemical shifts within a pre-chosen frequency range only. We have demonstrated the performance of this sequence versus frequency and its dependence on the different experimental parameters (e.g. the number of the DANTE pulses, n, and the rf strength, n1) and compared our experimental results to simulations. We found out that high selectivity is obtained when larger n is used, in agreement with the known DANTE performance.
A special case of the dbFSR experiment with n=2 was devised for the case of two particular chemical shifts of the recoupled nuclei. These experiments demonstrated the high selectivity and spectral editing capability embedded in this experimental scheme. The recoupling efficiency of the dbFSR experiment was found to be higher than that of the frequency-selective TEDOR experiment, however, the sensitivity of dbFSR experiment was found lower than that of the REDOR experiment. As an application, the dbFSR technique along with other 1D-techniques such as interrupted decoupling, REDOR, and frequency-selective TEDOR, were used to obtain partial spectral assignment of polymorph C of MeDNBP.
Another subject of investigation of this work is the characterization of putative dynamic equilibrium processes between polymorphs A and B at the phase transition temperature. For this purpose, the 2D-exchange solution NMR experiment was adopted and implemented as a CPMAS version, and applied to MeDNBP, in which both nitro groups are 15N labeled. Observing the 15N nuclei, below, at and above the phase transition temperature, with different mixing times of 30, 60, and 180 s, showed no evidence for the dynamic process A ↔ B; no inter-polymorph crosspeaks were observed implying that if such processes occur, their timescale is on the order of 30 min or longer.